[PATCH 6.6.y] memfd: deny writeable mappings when implying SEAL_WRITE

[Sasha Levin <sashal@kernel.org>]

From: "Pratyush Yadav (Google)" <pratyush@kernel.org>

[ Upstream commit 3b041514cb6eae45869b020f743c14d983363222 ]

When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X.  But the
implied seal is set after the check that makes sure the memfd can not have
any writable mappings.  This means one can use SEAL_EXEC to apply
SEAL_WRITE while having writeable mappings.

This breaks the contract that SEAL_WRITE provides and can be used by an
attacker to pass a memfd that appears to be write sealed but can still be
modified arbitrarily.

Fix this by adding the implied seals before the call for
mapping_deny_writable() is done.

Link: https://lore.kernel.org/20260505133922.797635-1-pratyush@kernel.org
Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Jeff Xu <jeffxu@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/memfd.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memfd.c b/mm/memfd.c
index 187265dc68f5e8..16dde40afd324b 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -210,6 +210,12 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		goto unlock;
 	}
 
+	/*
+	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
+	 */
+	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
+		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
+
 	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
 		error = mapping_deny_writable(file->f_mapping);
 		if (error)
@@ -222,12 +228,6 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	/*
-	 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start.
-	 */
-	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
-		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
-
 	*file_seals |= seals;
 	error = 0;
 
-- 
2.53.0